The present invention relates to a method of manufacturing a screen product, and to a screen product manufactured according to the method. The invention relates in particular to manufacturing a screen or a filter plate or a drum by a new machining method by which a product is produced the flow properties of which are remarkably better than the ones of prior art apparatus. The screen plate or cylinder of the invention is especially well applicable in the screens, filters, thickeners, washers, etc. used in the wood processing industry, but of course it can be applied also in sorting and thickening purposes in any other industry.
There are known several different prior art methods of manufacturing the screen products mentioned above. Some of the oldest and still used methods are drilling to produce round holes, punching to form apertures of the desired configuration, and milling to produce narrow slots. U.S. Pat. Nos. 239,837, 1,467,758, and 1,928,216 may be mentioned as examples of the above methods of manufacturing a screen plate. The mechanical machining methods mentioned above have for a relatively long time been the only applicable methods even though they have their drawbacks. Punching is usually out of the question when producing screen plates for the wood processing industry because the slot size required in the screens is so small that punching does not produce it without difficulty. Drilling can produce holes which are just about small enough. Drilling small holes is effectively limited by the tendency of the drills to break. Therefore, when a plate is manufactured by drilling, it is necessary either to mill or drill a larger opening in the plate into which opening the relatively small apertures produced by drilling open. This procedure reduces the tendency of the drills to break. However, the drawbacks of the increased number of work phases and the burrs or fins left in the edge of the aperture machined last, which is typical of mechanical machining methods, still remain. The fibers of the fibre suspension are easily caught by these burrs which gradually causes clogging of the screen. It is often very difficult to remove the burrs because they in most cases are located at the bottom of very narrow grooves or apertures the diameter of which is very small.
The same problems are met with also in screen products made by milling where the burrs remain at the bottom of the so-called back grooves. Because of the strength requirements of the product the back grooves cannot be made much wider than the slots extending through the plate. In most cases there is one screening slot per one back groove. This results also in that the open surface of the plate is limited mostly by the size of the neck surfaces required for the strength reasons between the back grooves both in the lateral and in the longitudinal direction of the grooves. Further, in the slots, more precisely in the side surfaces of the slots made by machining there are, of course, because of the machining method, small ridges almost parallel with the longitudinal direction of the slot which remarkably impair the flow through the plate. It is very difficult to eliminate this kind of factors affecting the quality of the surface and the capacity of the screen. This is, however, tried by finishing the plate after the machining, the purpose of the finishing being to remove a thin surface layer from all over the plate, both from the surface of the plate and from the apertures. However, it has been proved that this kind of finishing is not adequate to remove the burrs, but the screen plate industry all over the world is continuously looking for new methods of removing the burrs from the bottom of the narrow grooves. Further, the finishing unavoidably results in an increase in the size of the apertures in the plate which cannot always be taken into account in advance when the plate is originally being machined. The sizes of the apertures in the screen and sieve plates usable in the pulp industry are the smallest and only just about attainable by the mechanical machining method and they should not be enlarged from that size. This means that the machining blades used must continuously be in a very good condition so that the quality of the machined surfaces they produce is as good as possible. This results in increased maintenance costs of the machining devices and the manufacturing costs of the screen products increase correspondingly.
When considering a functionally optimal plate, the manufacture of the PROFILE.TM. screen plate developed by A. Ahlstrom Corporation and protected by for example U.S. Pat. No. 4,529,520 may be considered as one of the technically most difficult and non-economical machining object which requires locating the screening aperture either exactly at the root of the ridge in the plate or a little outward from the root of the ridge. Both drills and milling blades are very easily destroyed if the aperture is machined quite at the root of the ridge.
It is typical of both the machining methods discussed above that the diameter/width of the produced aperture is the same until the back opening/groove which in turn is connected to the aperture via a clear shoulder. As an example, a slot of 0.20 mm connected to a back groove of 1-2 mm may be mentioned. Thus the result is by no means a rheologically efficient flow passage. Further, it should be noted that, in the treatment of waste paper pulp, the corner between the back groove and the aperture/slot gathers waxes, adhesives and other sticky substances which first fill the corner and gradually the whole back groove. When studying prior art plates it has been shown that the accumulation of the sticky substances does not by any means stop when the corner is filled up to the aperture/slot but continues uninterrupted until the whole slot is clogged at the back.
European patent application EP 0 414 119 A1 dicloses a screen drum made of a screen plate having grooves on the inlet side of the screen drum. Apertures opening into the grooves are machined through the plate material. The apertures show a two step design, having a small diameter portion opening into the grooves and a larger diameter portion opening into the outlet side of the screen drum. Clearly at least two machining steps are needed to make the apertures, one step for machining a rather large diameter slot or bore on the outlet side of the plate and another step for machining, drilling, the small diameter screening aperture connecting the groove with the large diameter slot or bore. A shoulder is formed in the junction between the two portions. This screen drum and its manufacture are afflicted with the same drawbacks as mentioned earlier for screens having separate backgrooves machined therein. Laser cutting is worth mentioning as a more modern and somewhat more advanced manufacturing method which can be used in machining both apertures and slots. As is known, a laser beam melts material with the result that the walls of apertures manufactured by machining with a laser are hardened and also the material surrounding the apertures is subjected at least to a certain degree to a heat treatment which causes deformation of the surface which results in an increase in the flow resistance caused by the surface. An aperture made by a laser is practically speaking direct, only very minimal tapering is to be seen, the aperture opening in the direction of the beam, and the angle of taper being only 0.degree.-2.degree.. Further, it is typical of an aperture manufactured by a laser that the attainable minimum width/diameter of the aperture is directly proportional to the thickness of the plate. For example the narrowest slot attainable in a 6 mm thick plate, which is generally used in screen plates, is approx. 0.35-0.4 mm which is too large for most screening and thickening purposes. In order to have the flow resistance and the size of the aperture within an applicable and acceptable range the back groove manufactured by milling or drilling is still required to reduce the thickness of the plate to a be suitable for the laser cutting. Thus, as many work phases are still needed as by the conventional milling/drilling method mentioned above. The use of laser also produces burrs in the edge of the aperture which are almost as detrimental as the burrs in milled plates. The burrs should be removed from the edge of the aperture but it is difficult as the burrs are composed of a re-solidified material the removal of which is more difficult than the removal of the burrs produced by milling. Economical use of laser equipment is limited by the fact that only one cutting beam can be used at a time. Further, the maintenance costs of laser equipment are high.
Laser cutting further has the drawback of very strong local heat input in the workpiece which creates an internal stress peak in the material at the end of the slot cut. This residual stress has resulted in breaking of screen drums manufactured by this method, particularly of screens used in conditions of strong fluctuations in the dynamic load.
International patent application WO 82/02345 discloses a screen plate, in the construction of which the problems caused by milled back grooves have been taken into account. It is a characteristic feature of the embodiment of the publication that the thickening slot ends at the filtrate space side in a duct opening in the configuration of a V or in a duct having even curved walls. However, the publication does not describe the manufacturing method at all. It is obvious that during the time of the patent application, the problem of the back grooves was recognized and a solution was sought to it. A solution was found but no industrially applicable manufacturing method. When thinking about the manufacture of the plate in question the result is that, in order to have a functionally efficient end product, the unavoidable requirement is that the filtering slot and the back groove are machined from different sides of the plate. This in turn requires that either the slot or the groove must be made precisely, with the accuracy of tenths of a millimeter, in the correct place to produce an aperture of the correct configuration. In practice this is not possible, or at least economically possible. Thus the publication has remained as a sole attempt without any real industrial applications.
As can be seen from the above, the following drawbacks are typical of the manufacture of most of the prior art screen plates:
many work phases; PA1 the rheologically unfavourable configuration of the flow opening of which it is characteristic that it is formed of a screening small aperture and of a wider portion allowing a more free flow therethrough; PA1 a back groove/recess collecting sticky substances; PA1 a limited open area which depends more on the dimensioning of the back grooves/recesses than on the dimensioning of the screening aperture itself; PA1 heat stresses by some manufacturing methods; PA1 in most cases an aperture located perpendicular to the plate.
a) machining the back groove PA2 b) machining the screen aperture, and PA2 c) removal of burrs
It is possible with the present invention to eliminate the drawbacks described above of the prior art solutions and at the same time the configuration and the direction of the screen aperture can be optimized so that the aperture causes as little flow resistance as possible with the result that the capacity of both each of the openings and the entire screen plate substantially increases compared to prior art solutions. Further, the machining of the screening apertures can be carried out as one single work phase bacause the back groove and the after treatment is not necessarily needed.
Also, the manufacturing method according to the invention provides a possibility to produce apertures and slots with a free geometric configuration, or different combinations of these, the manufacture of which is at the present very difficult, practically impossible. It must also be noted that the cross sectional axis of the aperture or the slot need not be perpendicular to the plate to be cut.
It is characteristic of a preferred embodiment of the method of the present invention that it employs waterjet cutting in the manufacture of the screen products.
Until now the waterjet cutting mentioned above has been used mainly in the aircraft industry. Its use is relatively common in the food, plywood and plastics industry where the product to be cut varies from ice cream to dashboards of cars. It has also been used to some extent in the cutting of the printed circuit boards used in the electronic industry.
However, it must be stated that the use of a waterjet particularly in cutting applications is rapidly expanding and new areas of application are found for it all the time. These are for example the cutting of materials, which are by other methods difficult to cut, so that a piece of a desired form is cut from a plate. However, up to our invention, the waterjet cutting has been used to create a rectangular cut, in other words it has always been strived for an as straight slot as possible, with a rectangular cross section. All the studies and the development of the apparatus and its operation parameters have until now had as the goal to maximize the outcome of the cut, i.e. to produce rectangular cutting edges.
In an ordinary waterjet cutting, the pressure of clean filtered water is raised to approx. 1400-4500 bar and the water is supplied in a stainless steel piping to a nozzle, made of artificial sapphire and having a hole diameter of 0.1-0.6 mm so as to create a jet velocity of 600-2000 m/s. The water consumption of this kind of an apparatus is approx. 0.5-10 liters per minute. The distance of the nozzle from the workpiece is usually about 0.2-25 mm; however, the distances of 5-20 mm are the most recommendable. The nozzle is usually controlled by a robot or a corresponding means but small workpieces can be taken care of also manually.
Waterjet cutting can by used in the cutting of almost any conceivable material from paper, fabrics, plywood to composites, multilayered materials, ceramics and different hard metals. Also the food industry has found good applications for the waterjet cutting. In case of soft materials, only water is used as the cutting substance. When cutting metal and other hard materials, abrasive particles, such as granite, aluminium oxide, silicon nitride, olivine or other like material, is added to the water. In this case, about 90% of the cutting is considered to be effected by the abrasive substance and only 10% by the water.
It is charactristic of the method of our invention that apertures widening steplessly are machined in the plate material in one work phase and that the surface of the plate facing the material to be treated is provided with grooves in the bottom area of which said apertures open.
It is characteristic of the screen product of our invention that the apertures widening steplessly in the direction of the flow through the screen are located in the bottom area of the grooves provided in the surface of the plate.
It is characteristic of the use of the screen product according to our invention that the ratio of the cleanliness and the capacity of the screening operation is optimized by changing the angle of inclination of said apertures.
The invention is described below more in detail with reference to the accompanying drawings of which